1. Field of the Invention
The present invention relates to an imprint stamper for manufacturing a magnetic recording medium, a method for manufacturing the imprint stamper, a magnetic recording medium, a method for manufacturing the magnetic recording medium, a method for recording/reproducing information, and an information recording/reproducing apparatus.
2. Background Art
In recent years, exponential improvement of functions in information equipment such as personal computers has brought about more and more expectations on information recording/reproducing apparatus having high recording density. In order to improve the recording density, the dimensions of a recording material of each recording cell or each recording mark serving as a write unit for recording in a recording medium have to be made very small.
Generally, a polycrystal having a wide granular variation of single crystals is used for a recording layer of a magnetic recording medium such as a hard disk. Due to thermal fluctuation generated in the single crystals, recording becomes unstable in smaller single crystals which are more affected by the thermal fluctuation. Therefore, in a small recording cell, recording becomes unstable enough to increase noise. This is caused by the reduced number of crystals included in each recording cell and the relative increase of interaction among recording cells.
In order to avoid these problems, patterned media for performing recording/reproducing have been proposed in the field of magnetic recording (for example, see S. Y. Chou et al., J. Appl. Phys., 76 (1994) pp. 6673). In the patterned media, a recording material is segmented in advance by a non-recording material so that each single particle of the recording material is used as a single recording cell.
The method for producing such patterned media includes an artificial drawing method using light or electron beams or a method for producing a recording material out of a self-assembled material such as a diblock polymer or microparticles. According to the former method, high precision and high recording density media can be produced, but it takes more time than according to the latter method. On the other hand, according to the latter method, high-density media can be produced in a short time, but the accuracy deteriorates in comparison with that according to the former method (for example, see JP2001-279616 (kokai)).
When recording/reproducing is performed on a patterned medium, a recording/reproducing head having a recording head and a reproducing head runs on the medium. The recording/reproducing head is positioned using a positioning signal recorded in a servo region on the medium. This positioning allows the recording/reproducing head to gain proper access to each recording dot on the medium and perform recording/reproducing thereon.
A magnetic recording medium having a high recording density data region needs a high-precision servo region. In the aforementioned method for producing a patterned medium using a self-assembled material, the expected recording density of the recording medium is about 200 Gbpsi or higher, and the expected diameter of a recording dot is about 100 nm or smaller. Thus, the positioning accuracy of about 10 nm or lower is required.
However, a dot-shaped (granular) recording material is not suitable for drawing a minute pattern such as a positioning signal in the servo region. In addition, when the servo region is drawn, it is difficult to attain higher positioning accuracy by use of a material as large as the recording material in the data region than that of the recording material. Further, when the self-assembled material is used, there occurs a misalignment or a dimensional divergence in each dot due to fluctuation of the material itself or fluctuation in annealing. Due to the misalignment or the dimensional divergence, existing self-assembled materials are not suitable for the positioning signal in the servo region which signal needs high accuracy. It is therefore desired that the positioning signal in the servo region is created using the artificial drawing method.
The present inventors discovered the following problem in the attempt to mix both the methods in a single substrate in consideration of the aforementioned situation.
That is, according to the existing method for producing a patterned medium using a self-assembled material, it is possible to create a pattern all over the medium, but it is difficult to create a pattern in only a part of the medium or particularly in a region segmented at a minute interval. Therefore, when the self-assembled material is applied after a positioning signal is drawn in the servo region, the servo region attempted to be created by artificial drawing is also covered with the pattern using the self-assembled material. As a result, an unnecessary dot appears in the servo region so that the accuracy of the positioning signal in the servo region deteriorates.
On the other hand, when the positioning signal in the servo region is to be created after the pattern using the self-assembled material is formed in advance, the material of the servo region portion on the substrate must be a material withstanding high temperature in annealing in the process of self-assembling. Such a material is not suitable for artificial drawing in the following process.
According to the existing techniques, it is therefore difficult to manufacture a magnetic recording medium having a high-density data region and a high-precision servo region.
When a recording medium is manufactured with different methods being applied to the servo region and the data region respectively, there occurs a relative misregistration between the two regions. For example, assume that a magnetic recording medium is manufactured by forming the servo region in an artificial drawing method and forming the data region in a method using a self-assembled material. Then, each method is carried out with a predetermined reference point. Since the reference point is fixed to one and the same position on a substrate, positioning is required. The accuracy of this positioning has a limit due to mechanical reproducibility, uneven expansion caused by uneven temperature, mixture of impurities, etc. Thus, there occurs a misregistration between the servo region formed in the artificial drawing method and the data region formed in the method using a self-assembled material.
Examples of positioning methods in the background art include a physical method, an optical method, etc.
In the physical positioning method, each subject is brought into contact with a special guide structure so as to be positioned. However, due to an error caused by the thickness of a layer of the air between the structure and the subject, there is a limit of about 10 μm in the positioning accuracy. On the other hand, in the optical positioning method, optical marks drawn in two subjects are aligned so that they are positioned. However, due to an error caused by the wavelength of light, there is a limit of about 50 nm in the positioning accuracy. Also by use of electron beams, there is a limit of about 15 nm.
In conclusion, the limit of about 10 nm or lower in the positioning accuracy cannot be satisfied in either positioning method. For example, assume that a plurality of different methods as described above are mixed on a single substrate so as to manufacture a recording medium having a high recording density of about 200 Gbpsi or higher. In this case, the relative misregistration occurring among pattern regions cannot be corrected.